The invention is generally directed to the field of semiconductor manufacture and, more particularly, to a method of making accurate, reliable and reproducible semiconductor surface characterization measurements, including identifying surface anomalies such as dishing and erosion regions, notwithstanding the presence of noise signals in the surface characterization map.
In semiconductor fabrication, there is an ever-present need for methods to further improve reliability, yield and cost.
Semiconductor manufacturing processes includes the steps of, for example, etching a plurality of spaced-apart trenches into a surface layer of a conventional dielectric material such as a silicon-based wafer. Once the trenches are formed, the process typically includes applying or plating, on the surface layer, a layer of an electrically-conductive metal such as copper, which also fills the trenches. The trench-filled and metal-covered surface of the dielectric wafer is subsequently polished, typically by a conventional process known in the art which employs a known form of chemical mechanical polish, down to the dielectric layer.
The dielectric layer, typically an oxide, is not as easily polished away during the chemical mechanical polishing process as the surface-deposited, trench-filling metal, principally because the metal is xe2x80x9csofterxe2x80x9d than the oxide. As a result, the oxide surface tends to serve as a mechanical xe2x80x9cstopxe2x80x9d during the chemical mechanical polishing process. Metal remaining in the trenches thus forms a pattern of conducting paths. Note that the term xe2x80x9cdielectric,xe2x80x9d as used herein, is to be understood to mean a substance which contains few or no free electrons and which has an electrical conductivity that is so low as to be considered an insulator.
One problem encountered in the above-described semiconductor manufacturing process is known as xe2x80x9cdishing,xe2x80x9d which occurs when a pad, used in the chemical mechanical polishing process, deforms into the metal-filled trench as a result of pressure applied by the pad in conjunction with the resistance presented by the oxide surface. As is appreciated by those skilled in the art, the depth of dishing into a trench may be deeper for wider trenches. Notably, anything other than minimal dishing is generally undesirable, since the result may adversely affect the desired electrical properties and/or functions of the metal deposited in the trench.
Another problem that may be encountered in conventional semiconductor manufacturing processes is xe2x80x9cerosionxe2x80x9d which occurs when a pad, used in the chemical mechanical polishing process, wears away some of the oxide surface as a result of the pressure applied by the pad opposite the oxide surface. It can be well appreciated that erosion is particularly undesirable for multiple alternating layers (along the semiconductor surface) of metal and dielectric material, as erosion of the dielectric material increases the risk of a short between adjacent metal layers. Thus, erosion is particularly problematic in semiconductor wafer structures having a relatively high number of tightly-packed metal-filled trenches with relatively thin walls of dielectric oxide wafer material between adjacent metal-filled trenches.
Similarly, in the event that the trench filling metal is harder than the oxide, the xe2x80x9ceroded areaxe2x80x9d can actually rise above the oxide surface, according to a phenomenon known as xe2x80x9cnegative erosion.xe2x80x9d More particularly, in this case, the polishing process removes the oxide faster than the metal due to the metal being generally harder, causing dishing in the oxide and the removal of the substrate xe2x80x9csurface areaxe2x80x9d (see, for example, 18 in FIG. 3, discussed below) faster than the alternating metal layers, thus compromising the desired planarity of the resulting semiconductor surface.
Overall, erosion in conjunction with dishing may further adversely affect desired electrical properties and/or functions of the metal deposited in the trenches. In general, it is desirable for a semiconductor manufacturer to know when dishing and/or erosion is occurring, as well as the rate and amount of such dishing and/or erosion. Accuracy and precision, when locating the semiconductor upper surface as well as the bottom of dips due to dishing and erosion, must be statistically satisfactory, reliable and reproducible. Conventional methods are not.
A problem introduced when attempting to characterize the dishing and erosion phenomena is xe2x80x9cnoise.xe2x80x9d Noise problems occur, for example, when dust and other air-borne and/or electrically-charged particles adhere to the semiconductor surface. In the context of the preferred embodiment, the xe2x80x9cnoisexe2x80x9d-based problem affects the accuracy and efficiency of the dishing and/or erosion measurements. For example, while the noise-causing particles are often microscopic, it is important that a typical surface scan profile may include a total distance of about 2-5 millimeters along the semiconductor surface, involving perhaps 200-250 thousand points or xe2x80x9careasxe2x80x9d of interest (or xe2x80x9cregionsxe2x80x9d), wherein a vertical depth measurement for xe2x80x9cdishingxe2x80x9d purposes may be about 150-200 nanometers, and a typical vertical depth measurement for xe2x80x9cerosionxe2x80x9d purposes may be about 30-40 nanometers, wherein both depth measurements are made relative to the semiconductor surface.
One current method of profiling and characterizing a semiconductor surface after the chemical mechanical polishing procedure, includes scanning across a sample surface of the semiconductor with a conventional metrology instrument, and then generating a plot or map of the data. Such plots are typically presented to a semiconductor-manufacturing operator for analysis.
Conventional statistical averaging of the data, which attempts to correct for any noise that may be present, has not yet resulted in statistically satisfactory accuracy and precision, nor the attendant reliability and reproducibility of the semiconductor characterization information that is currently being sought by many semiconductor manufacturers. One such method averages the metrology data, including the noise signals, in an attempt to accurately determine the peaks. The averaging method is unreliable because it introduces error when noise signals are averaged.
Another method involves utilizing percentiles of the measurement data, including noise signals, in an attempt to determine peaks corresponding to dishing and erosion regions. The percentile method, unreliable because, like the averaging method, the noise signals must be accounted for when determining surface anomaly information, is not readily reproducible for the reason that an operator must exercise judgment regarding what percentile value to set any particular reading. The operator typically selects a level above or below which a certain percentage of the surface characterization points occur. For example, if the operator selects a particular depth, the percentile method may determine that 95% of the points are above that depth, thus indicating an extreme depth. However, in this example, the issue becomes whether the xe2x80x9c95% levelxe2x80x9d corresponds to the low peak, indicating that the other 5% of the points may correspond to, for example, noise, or whether the level should be set lower to xe2x80x9ccatchxe2x80x9d the peak. Clearly, this involves some guess work on the part of the operator, and often times will require some quantifying of the noise present in the data.
In some known scanning operations, information is obtained, stored and analyzed regarding the top surface (or reference) of the sample surface as well as deviations (e.g., dishing and erosion data) therefrom and noise information is extracted. FIG. 1 illustrates typical topography data resulting from a scan of a semiconductor sample, and in particular, dips and spikes due to noise. The topography, and thus the noise signal (N.S.), runs from left to right along the scan direction (S.D.), as shown. Several spikes (S1, S2, S3, S4) extend upwardly from the smaller noise signals, and dips (D1, D2, D3) extend downwardly. Noise affects determination of the xe2x80x9cactualxe2x80x9d surface, as influenced by noise, is illustrated in FIGS. 2A and 2B, depicting actual surface (FIG. 2A) and probability (FIG. 2B).
In particular, for a perfectly flat reference surface (R.S.), for reasons mentioned above, the use of conventional surface determination methods will typically result in there being a noise signal (N.S.) which is spaced above (A) or below (B) the reference surface, as is shown. As appreciated by those skilled in the art, noise may arise from xe2x80x9cactualxe2x80x9d or xe2x80x9ctruexe2x80x9d defects (e.g., cracks, pits and ridges) as well as xe2x80x9cfalsexe2x80x9d defects (e.g., adhered particles) along the surface of the semiconductor scan region. Therefore, to investigate many such noise signals, conventional methods and techniques are frequently employed to generate a probability curve (P) (FIG. 2B), that is based upon the noise signals, for the purpose of producing statistically reliable xe2x80x9cmost likelyxe2x80x9d data relative to xe2x80x9cactualxe2x80x9d or xe2x80x9ctruexe2x80x9d location of the reference surface. For example, conversion of the noise signals into digital data may result in the production of the probability curve (P).
With further reference to FIGS. 1A and 1B, and as is well known for so-called xe2x80x9cnormalxe2x80x9d distribution models, will result in the so-called xe2x80x9cTxe2x80x9d distribution being used statistically to verify the xe2x80x9cactualxe2x80x9d or xe2x80x9ctruexe2x80x9d location of the reference surface of the semiconductor. Further in that regard, a variety of other statistical models are well known (e.g., Gaussian distribution, Poisson distribution, the so-called xe2x80x9cFxe2x80x9d distribution, Chi-squared distribution, Hypergeometric distribution, and so forth). Such and other statistical models may be used, and frequently are used, by those skilled in the art. Generally, those employing such statistical methods are known to use xe2x80x9cstandardizedxe2x80x9d tabulated data to verify that information of concern to the semiconductor manufacturer appears in the xe2x80x9cone minus alphaxe2x80x9d or central region of the probability curve (P) and not along the so-called xe2x80x9cone-half alphaxe2x80x9d or trailing-edge margins of the curve, as is depicted in the plot of FIG. 1B.
With continued reference to FIG. 1, spikes pose a special problem, as many spikes are known to arise from a single-point surface defect, generally with no immediately-surrounding surface region information being present to indicate as to whether the defect is actual or xe2x80x9cfalse.xe2x80x9d Conventional methods and techniques to account for spikes may result in averaging-in false information or disregarding xe2x80x9cactualxe2x80x9d or xe2x80x9ctruexe2x80x9d information, either of which impacts the value of the information that results. In particular, known systems that minimize or otherwise quantify this noise data with such complex methods are computationally intensive, and are relatively imprecise according to present standards.
As noise introduces uncertainty into measurements involving, for example, the subtraction of a dish and/or erosion depth location from a semiconductor surface location, it would therefore be desirable to be able to minimize or otherwise eliminate the effects of noise from such semiconductor characterizing measurements. High accuracy, reproducibility and reliability of the data should be assured so as to introduce a higher degree of certainty into the measurements. Therefore, the art of characterizing semiconductor surfaces was in need of a method that identifies surface anomalies, including dishing and erosion data, and characterizes the anomalies with respect to amount and rate of occurrence. Further, the method should determine the surface anomaly information in a reliable and in a readily reproducible manner, independent of the negative effects due to noise signals in the surface measurements.
One object of the present invention is to provide a method that enables a semiconductor manufacturer to determine an amount of dishing during process.
Another object of the present invention is to provide a method that enables a semiconductor manufacturer to determine the rate of dishing.
Yet another object of the present invention is to provide a method that enables a semiconductor manufacturer to determine the amount of erosion during process.
Still another object of the present invention is to provide a method that enables a semiconductor manufacturer to determine the rate of erosion.
A further object of the present invention is to provide a method that enables a semiconductor manufacturer to minimize or eliminate the effect or noise signals on the semiconductor characterizing measurements, for assuring high accuracy, reproducibility and reliability of the data, thereby introducing a high degree of certainty into the measurements.
The preferred embodiment of the present invention determines surface anomaly information, particularly dishing and erosion information relating to semiconductor manufacture, by virtually eliminating the effects of noise from the determination of dishing and erosion. The method takes advantage of the fact that the surface characterization data corresponding to either surface regions or anomaly regions will be much more frequent than individual occurrences of noise associated with the topography data of the sample surface.
According to a first aspect of the preferred embodiment, a method of characterizing a sample surface having a surface anomaly region includes the steps of profiling the sample surface to generate surface characteristic data, and generating a histogram based on the profiling step. Then, the method measures a surface anomaly in the surface anomaly region based on the generating step.
According to a further aspect of the preferred embodiment, this method includes the step of selecting a zone of interest from the surface characterization data. The zone of interest preferably includes the surface anomaly region, wherein the surface anomaly region includes one of erosion and dishing.
According to yet another aspect of the preferred embodiment, the histogram includes a first peak corresponding to a generally planar portion of the sample surface, and a second peak corresponding to the surface anomaly. Further, the measuring step includes determining a distance between the first and second peaks, the distance being indicative of the depth of the surface anomaly.
In a still further aspect of the preferred embodiment, a method that measures dishing values and erosion values associated with surface topography data generated by scanning a semiconductor surface includes the steps of: (A) generating a histogram of a portion of the surface profile data corresponding to a first zone of interest; and (B) smoothing the histogram of the generating step to produce a smoothed curve having a peak corresponding to one of a dishing value and an erosion value.
According to another aspect of the preferred embodiment, the first zone of interest includes dishing and erosion data, and the smoothed histogram includes first, second and third peaks corresponding to a reference surface, an erosion value and a dishing value, respectively.
In a still further aspect of the preferred embodiment, a method for measuring dishing values and erosion values of a semiconductor surface by scanning the surface to obtain surface profile data that contains either dishing data or erosion data or dishing and erosion data, all referenced to surface data, includes the steps of leveling the surface profile data and generating a histogram of a portion of the leveled surface profile data corresponding to a first of a plurality of zones of interest. Then, the method includes smoothing the histogram of the generating step to produce a smoothed curve having a maximum value corresponding to an erosion value or a dishing value. Finally, the method includes repeating the generating and smoothing steps relative to each of the remainder of the plural zones of interest to produce smoothed curves corresponding to an erosion value or a dishing value or both for each of the remainder of the plural zones of interest.
These and other objects, features, and advantages of the invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.